1. Field of the Invention
The present invention relates to an impact modifier for a thermoplastic resin having an excellent heat discoloration resistance and heat deterioration resistance under high-temperature conditions, and a thermoplastic resin composition formed by incorporating this impact modifier into a thermoplastic resin.
2. Description of the Related Art
Currently a polyester resin such as polyethylene terephthalate or polytetramethylene terephthalate is used for parts and members subjected to high temperatures, such as oven-useable trays, engineering plastics for exterior parts of heating devices, and electric parts, by utilizing the high melting points of such a resin. These polyester resins moreover have a poor toughness, and accordingly, a method has been proposed in which a methyl methacrylate/butadiene/ styrene copolymer (MBS resin) comprising a butadiene polymer as the rubber component is incorporated into a polyester resin to improve the impact resistance of the polyester resin.
A polycarbonate resin has an excellent impact resistance, heat resistance, dimensional stability, and electric insulating property and is widely used for machine parts, safe tools, electrically insulating members, optical machine parts, and containers.
This polycarbonate resin however, has a poor flowability and chemical resistance, and accordingly, as a means of overcoming there defects, a polymer-alloying process has been proposed and developed in which a saturated polyester resin such as polyethylene terephthalate or polytetramethylene terephthalate is incorporated in the polycarbonate resin (see Japanese Examined Patent Publication No. 36-14035, Japanese Examined Patent Publication No. 53-12537 and Japanese Examined Patent Publication No. 57-2744), and thus the application range thereof has been broadened. Nevertheless, if a saturated polyester resin is incorporated in a polycarbonate resin, the toughness possessed by the polycarbonate resin is reduced and accordingly, a process has been proposed in which the impact strength is improved by further incorporating an MBS resin (see Japanese Examined Patent Publication No. 55-9435).
It is known that an MBS resin can be incorporated in a vinyl chloride resin, and it is known as well that an MBS resin can be incorporated into a thermoplastic resin such as a saturated polyester resin or a polycarbonate resin, to improve the impact resistance.
But where a thermoplastic resin, in which an impact modifier comprising a butadiene polymer as the rubber component is incorporated is subjected to extrusion molding or injection molding under high-temperature conditions, the physical properties are often reduced by a heat discoloration or heat deterioration attributed to the butadiene rubber component in the MBS resin. Furthermore, this impact modifier is disadvantageous in that, if the obtained molded article is heat-treated, the impact strength retention ratio is lowered, and thus various improvements have been proposed; for example, an incorporation of a phenol type heat stabilizer into a thermoplastic resin. More specifically, the incorporation of a phenol type heat stabilizer is disclosed in the specification of U.S. Pat. No. 3,637,582, the incorporation of athioether type heat stabilizer is disclosed in U.S. Pat. No. 3,758,549 and the incorporation of a phosphite type heat stabilizer is disclosed in U.S. Pat. No. 4,057,163, U.S. Pat. No. 4,088,709 and U.S. Pat. No. 4,474,914. Furthermore, a combined incorporation of a phenol type heat stabilizer and a thioether type heat stabilizer is disclosed in U.S. Pat. No. 4,105,629, U.S. Pat. No.4,507,417, and U.S. Pat. No. 4,576,734 and the combined incorporation of a phenol type heat stabilizer and a phosphite type heat stabilizer is disclosed in U.S. Pat. No. 3,595,936, U.S. Pat. No. 4,025,486, U.S. Pat. No. 4,472,548, and U.S. Pat. No. 4,584,146.
In these proposals, however, where a thermoplastic resin, in which an impact modifier comprising a butadiene polymer as the rubber component is incorporated is subjected to extrusion molding or injection molding under high-temperature conditions, a reduction of the physical properties by a heat discoloration or heat deterioration attributed to the butadiene polymer rubber component in the MBS resin, and a reduction of the impact strength retention ratio upon heat treatment of the obtained molded article, cannot be properly controlled.